The present invention relates to a wireless code-division multiple-access communication system in which multiple mobile stations communicate with a single base station, as in a cellular telephone system or personal communication system.
Code-division multiple-access (hereinafter, CDMA) is a type of spread-spectrum system that allows multiple stations to communicate over the same channel, A mobile communication system of the type described above has a single upstream channel on which all mobile stations transmit to the base station, and a single downstream channel on which the base station transmits to all the mobile stations. On each of these channels, each mobile station is assigned a different spreading code, which is used to spread that station's signal before transmission and despread the signal after reception, If the spreading codes are mutually orthogonal (if any pair of spreading codes has zero mutual correlation), and if all signals are mutually synchronized, then each mobile station's signal can be received without interference from other stations' signals.
In the upstream channel, however, synchronization of signals transmitted from different mobile stations is difficult to achieve, so these signals usually interfere with each other to some extent. This interference limits the number of mobile stations that can access the system simultaneously.
In the downstream channel synchronization is not such a problem, because all signals are transmitted by the same base station, but if there are many mobile stations, it becomes difficult to provide the necessary number of mutually orthogonal spreading codes. Very long spreading codes have to be used, with a high spreading gain (high chip rate), but this undesirably broadens the bandwidth of the downstream channel. The alternative is to employ spreading codes that are not all mutually orthogonal, accepting a certain amount of interference. In either case, the number of mobile stations that can access the system is again limited.
Although synchronization of different spreading codes presents no problem on the downstream channel, there is a problem of carrier synchronization when synchronous (coherent) detection is employed. In synchronous detection, a mobile station multiplies the received signal by a locally generated carrier signal which is synchronized in phase to the base station's carrier signal. The problem is in acquiring and maintaining phase synchronization as the mobile station moves in relation to the base station.
A further problem is that as a mobile station moves through electrically noisy areas or areas with obstructions to radio transmission, the rate of errors in the received signal inevitably rises. The signal transmitted by a mobile station is typically a voice signal that has been encoded by adaptive differential pulse-code modulation (ADPCM), using an adjustable scale factor to prevent overflow and underflow in the encoding process. Known methods of adjusting the scale factor work well at low error rates, but at high error rates they are found to produce spike noise in the received signal. Spike noise is highly obtrusive in voice communication, and seriously degrades the perceived voice quality, even if the signal-to-noise ratio is affected only slightly.